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Esters of 2-arylcyclopropanecarboxylic acids react with nitrous acid generated in situ with regioselective insertion of the nitrosyl cation into the cyclopropane ring. Depending on the substrate/nitrosylating agent ratio, the reaction proceeds with the formation of either aryl-substituted 3-ethoxycarbonyl-4,5-dihydroisoxazoles or the corresponding isoxazoles. The nature and position of the substituents in the aromatic ring of the starting 2-arylcyclopropanecarboxylic acid esters affect the reaction rate but have no effect on the regioselectivity of the attack by the nitrosyl cation on the three-membered ring. A dependence of the reactivity of isomeric substrates on their stereochemistry and position of the nitro group in the aromatic ring is noted for 2-and 4-nitrophenyl derivatives of esters of cis-and trans-2-arylcyclopropanecarboxylic acids.Keywords: 5-aryl-3-ethoxycarbonyl-4,5-dihydroisoxazoles, 5-aryl-3-ethoxycarbonylisoxazoles, ethyl esters of 2-arylcyclopropanecarboxylic acids, insertion of the nitrosyl cation into the cyclopropane ring.The reaction of arylcyclopropanes with nitrosylating agents, which proceeds through insertion of the nitrosyl cation or nitrosyl radical into the cyclopropane ring, may be seen as one of the most efficient methods for the synthesis of aryl-substituted 4,5-dihydroisoxazoles and isoxazoles, which are potential pharmaceutical agents and important precursors for the preparation of biologically-active compounds, including natural products [1][2][3][4][5][6].

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Mass spectrometry proved itself to be a powerful tool to predict the directions and yields of mono- molecular reactions of organic compounds. Electron ionization (EI) and electrospray ionization (ESI) were used to study possible transformations of N-(ortho-cyclopropylmethylphenyl)arylamides I and N-(ortho- cyclopropylmethylphenyl)arylthioamides II as well as their para-isomers III and IV in a mass spectrometer and to predict directions and yields of their acid catalyzed cyclization reactions. Several five-eight-membered heterocycles were proposed as possible products of intramolecular transformations of compounds I and II. Reactions of compounds I and II in sulfuric acid solutions were carried out and the results obtained were compared with mass spectrometric data. Surprisingly, EI of the studied compounds mimics their solution reactions better than ESI.

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